The goal of this revised grant proposal is to explore strategies to modulate the activity of the brain proteoglycan syndecan-3 for the therapy of cocaine abuse. Proteoglycans like syndecan-3, while originally identified as components of the extracellular matrix (ECM), also play signaling functions as receptors and co-receptors for growth factors and ECM proteins. We observed that syndecan-3 in the lateral hypothalamus (LH) has an unexpected new role in limiting compulsive cocaine intake. In particular, in Specific Aim 1 we will investigate syndecan-3 ectodomain shedding and signal transduction in the regulation of cocaine intake. The proteolytic cleavage of syndecan-3 ectodomain, also known as shedding is induced after ligand interaction and terminates its signaling. To probe the functional consequence of ectodomain shedding, we will use adeno-associated (AAV) viral vectors to deliver modified syndecan-3 constructs including a shedding-resistant syndecan-3, the syndecan-3 ectodomain alone, and the unmodified syndecan-3. The results of these studies will establish the therapeutic potential of inhibiting the proteolytic cleavage of its ligand-bindig ectodomain. Studies in Specific Aim 2 will investigate the role of syndecan-3 as an alternative GDNF receptor in the ability of syndecan-3 to reduce cocaine self-administration. To this end we will use AAV to transduce modified GDNF constructs designed to bind selectively either to syndecan-3 or to its canonical high-affinity receptor GFR-1. The results of Specific Aim 2 will inform future studies aimed at manipulating interactions between syndecan-3 and its two receptor systems and signal transduction partners to boost its signaling activity. Together, the proposed studies will increase our understanding of the functions of the hypothalamic proteoglycan syndecan-3 as a regulator of behavior and will explore innovative therapeutic strategies for the therapy of cocaine abuse.